The present invention relates to a color image forming apparatus wherein a plurality of electrostatic latent images formed on an image bearing member through an electrophotographic process or an electrostatic recording process, corresponding to separated color components, and are developed by a plurality of developing devices to provide a color image.
A color image forming apparatus has been put into practice wherein a color-separated image of an original is formed on an electrophotographic photosensitive member as a latent image, which is developed with a developing agent, and the developed image is transferred onto a transfer material, and those operations are repeated with developing agents having different colors, whereby a color image is formed on the transfer material.
This apparatus, however, involves a drawback in that the diameter of the photosensitive member or drum has to be large enough to allow three or for developing devices to be fixedly disposed near and around the photosensitive drum in order to provide a full-color image.
U.S. Pat. No. 3,987,756 has proposed that three or four developing devices for different colors are arranged about an axis so that they are as a whole rotatable about the axis in one direction, whereby a developing device corresponding to a latent image is moved to a developing station when it is to be used. This is advantageous because the diameter of the photosensitive drum may be made smaller. Therefore, the entire image forming apparatus may be made smaller, and in addition, the photosensitive drum may be a multipurpose one (a photosensitive drum is commonly usable for a black-and-white image formation and a multi-color image formation).
However, the mechanism described above is disadvantageous in that it requires at least 1-2 seconds to move, after a developing operation is carried out with a certain color, the next developing device to the developing station. If this time is reduced, the inertia force increases, resulting in a greater shock upon the stoppage of the developing device, which would disturb the image forming process including the latent image forming step even to such an extent that the quality of the image is remarkably degraded. Even if the speed of the movement is maintained, the above-described problems arise if the weight increases. For those reasons, there is a limit in reducing the time required for circulating developing device.
In the color image forming apparatus of this type, when, for example, three copies are taken in a color operation mode wherein only yellow and magenta developing agents are used, the cyan developing device and the black developing device which are not used have to be passed by and stopped at the developing station three times. This leads to slow image formation since the wasteful time is needed. Additionally, the distance through which the developing devices move is larger, resulting in an unnecessary vibration of the apparatus and an unnecessary noise produced.